1. Field of the Invention
This invention relates to logic circuits, and more particularly to a current mode logic circuit in which the DC levels of the output signals and input threshold are almost kept invariant irrespective of changes in temperature and supply voltage.
2. Description of the Prior Art
The conventional current switch circuit consists of a so-called reference transistor of which a reference voltage is impressed on the base and a so-called input transistor of which an input signal to the circuit is impressed on the base, both transistors being connected with the emitters common.
A typical circuit including the current switch circuit as the basic component is a circuit known as a CML (current mode logic) which includes a plurality of input transistors, the respective emitters as well as collectors of the input transistors being mutually connected respectively, so that an OR output can be taken out from the collector of the reference transistor and a NOR output from the common collector lead of the input transistors.
The CML in which the transistors are operated in the respective non-saturation regions so as not to be affected by the stored charge, is an effective logic circuit compatible with the so-called CTL (complementary transistor logic) especially in the operation speed. In the practical use of a CML, a further stage of an emitter follower is connected to each output circuit of the reference and the input transistors so as to reduce the output impedance and to equalize the levels of the input and output signals.
Conventional current switch circuits including the above-mentioned CML, however, have a common drawback that the DC voltage levels of the output signals and input threshold are changed by the temperature supply voltage.
The Variation of DC voltage levels and input threshold in temperature are caused by the temperature dependence of the forward base-emitter voltage, and the variation in supply voltage are caused by the biasing variation of a regulation transistor.
For example, with a typical practical CML which is designed so as to have a logic swing voltage of approximately 0.8 V with the "1" level of the output signal set at -0.9 V or so, the "0" level at -1.7 V or so and the reference level at -1.3 V or so, the "1" level signal is subject to the variations with a temperature coefficient of about 1.3 to 2.0 mV/.degree. C., especially under the influence of transistors of the emitter followers. As for the "0" level signal, the temperature coefficient of the drift is found to be about 0.5 to 0.8 mV/.degree. C. which is much lower than that for the "1" level signal. This is due to the fact that the "0" level signal is affected also by the temperature characteristics of the input transistors and reference transistors which more or less compensate for the influence of the drift of the emitter follower transistors.
Anyway, such temperature dependence of the output levels results in a poor immunity of the signal against noise in the conventional CML. Especially if the conventional CML is fabricated in an integrated circuit formation, such a circuit would sometimes fail to operate at the intended logic swing voltage, as the component circuit elements are not allowed sufficient heat dissipation. Further, the limit level of the input signal below which the logic circuit can operate in a non-saturation state, also varies with the temperature coefficient of about -1.5 to -1.8 mV/.degree. C. in a range of -0.4 to -0.8 V. Therefore, if the "1" level signal rises with a temperature rise, it is possible for the signal level to trespass on the saturation region, exceeding the above-mentioned limit level. This necessitates a more limited tolerance of the temperature characteristics of the component circuit elements.
It is, however, a modern trend to integrate the logic circuits to large scales accompanied by the generation of increased amount of heat, thereby presenting a problem of temperature dependence of the logic circuits.
To integrate the logic circuits to a large scale, therefore, they must operate on a power supply of a voltage which is as small as possible, and on input signals of small amplitude. However, there is posed a limit in regard to the voltage of the power supply and the amplitude of the input signals due to the temperature dependence of the output level and the voltage dependence of the power supply.